CAN YOU TELL ME WHY MY CAR IDLES AS LOW AS 5000RPM

1991 Mercedes Benz 230te • 168,000 miles

When I got the car 3weeks ago it idle at around 15000rpm depending on the weather when lookin at it the throttle cabble was adjusted to far an the throttle arm wasnt sitting on the idle switch in the book it says its ment to so I have ajusted it to where it should be now it idles at 7500rpm an in warmer weather it can drop to 5000rpm an try to stall can you help thank you